For vehicles in general, many different power train configurations are available. For example, the gearbox may take the form of a manually operated gearbox or an automatically operated gearbox. It is often desirable that heavy vehicles should be drivable as comfortably for the driver as possible. This means, for example, that the gear changes should be executed automatically by the control system usually incorporated in the vehicle. Automatically operated gearboxes have therefore become increasingly common in heavy vehicles.
However, this automatic gear changing is usually not executed by an automatic gearbox in the traditional sense, but by a “manual” gearbox controlled by the control system, partly because manual gearboxes are substantially less expensive to manufacture, but also because they are more efficient. With regard to automatic gearboxes of the type commonly used in passenger cars, the level of efficiency is often too low, compared with a manually operated gearbox, to justify their use other than in, for example, city buses and local delivery vehicles in urban areas where frequent starting and stopping is usual.
Heavy vehicles largely used on major roads/motorways therefore usually have automatically operated “manual” gearboxes.
This gear changing may be effected in several different ways. In one type the driver uses a clutch pedal to set the vehicle in motion from stationary, but all other gear changing can be effected by the vehicle's control system without involving the clutch at all. Instead, the gear changes are carried out “torque-free”, i.e. the torque delivered from the engine is adjusted to a suitable level to reduce the torque transmitted at the engagement points of the relevant gears.
Another method is to use instead an automatically controlled clutch with automatic upshifts/downshifts, in which case the driver has access to only an accelerator pedal and a brake pedal.
On this type of vehicle with an automatic clutch, just as in the case of a manually controlled clutch, the clutch must open when the driver presses the brake pedal and the vehicle's running speed has been slowed to a level at which the engine speed approaches the idling speed. Hard braking involves certain requirements for this clutch opening. If the clutch opens too quickly, there is risk of causing undesirable vibrations in the power train, whereas if the clutch opens too slowly, there is risk of the engine speed dropping to such a low level as to risk the engine stalling.
There is therefore a need for an improved method for opening of the clutch of vehicles with an automatically controlled clutch.